Introduction

Corg-Cycle

Microorganisms, particularly bacteria (=prokaryotes), represent the first sustainable system in the biosphere into which all other living beings are superimposed and included. Sustainability of the system depends on the catalytic role of bacteria in the cycles of biogenic elements and their mediating role in the transformation of other elements. Development of cooperative micro-bial community led to the biogeochemical succession, the most prominent result being oxygenation of the atmosphere around 2.4 billion years ago with interconnected changes for chemical compounds. The role of bacteria in the biosphere depends on their functional diversity and formation of cooperative trophic systems, scaling up from the local ecosystems to the biosphere as a whole. The limits of life are delineated by the topic adaptability of bacteria, while all other living beings remain within these frames. The number of bacteria exceeds 1028 in the active layers with rapid turnover and might be 1030 in total. It makes them by far the most important group in the conceptual structure of the sustainable biosphere. The trophic structure of the microbial system makes the framework of the biosphere. The interconnection of biogeochemical cycles makes the functional role of microorganisms in the biosphere most fundamental.

Biogeochemical cycles represent the main system by which the energy of the Sun is transformed into energy of the chemical compounds by living beings and products of their activity. The cyclic arrangement is the main principle of sustainability in the Earth system. It means that the compound involved in the process after sequential transformations is regenerated as its end product. Cycles are regarded as the cycles of the elements. Stepwise reactions of the cycles are catalyzed by specific groups of microorganisms. The system of higher organisms is superimposed into the initial cooperative system constructed by bacteria.

The driving force of the system of interlinked cycles is the cycle of organic carbon (Corg). The cycle involves two steps: production and destruction. During production CO2 is assimilated in the biomass; during destruction dead biomass is decomposed into CO2. Composition of biomass includes in addition to Corg, as the main components Norg and Porg in approximate molar ratio 106:16:1. This ratio calculated for marine phytoplankton is quoted as 'Redfield ratio'. H and O are included in the biomass in water in the ratio 2:1, making the reductive level of Corg close to [CH2O]. Strong deviations from Redfield ratio are known for terrestrial biomass with organic supportive structures as in trees with Corg:Norg about 500; minor deviations are caused by storage products. There are other elements included in the biomass such as Sorg, and a number of essential 'mineral' elements beginning with K, Fe, Mg, Ca, and microelements. Composition of the living biomass might be considered as invariable with minor deviations (Figure 1).

Transformation of CO2 into Corg is performed by auto-trophic organisms by the metabolic pathways where the Calvin cycle is quantitatively dominating; other auto-trophic reactions seem not important quantitatively on the global scale. The key enzyme is ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) which carboxylates phosphopentose regenerated in the cyclic metabolic pathway (Calvin cycle). Due to the discrimination of13C during autotrophic assimilation isotopically lighter carbon with

S 13Corg--25%% is produced, which is considered as the isotopic signature of a biotic source. Careful interpretation of isotope fractionation data is strongly recommended since they depend both on biotic pathways and inorganic diffusion. Assimilation depends on the source of energy, light of the Sun for photosynthesis, or oxido-reductive reaction of inorganic compounds for chemosynthesis (che-molithotrophy is the later synonym) in subterranean systems. In photosynthesis the overall reaction CO2 + H2O ! [CH2O] + O2 takes place. It makes a coupled cycle with an equimolar ratio of CO2/O2. The quantity of O2

Figure 1 Interlink between the cycles of the main biogenic elements. Cycle of Corg makes the main driving force of machinery coupled to the cycles of biomass constituents Norg and Porg and catabolic cycles of oxygen, sulfur, and iron. Cycles of these elements are coupled to reservoirs of inorganic matter in the geosphere. For each time period approximate material balance should be sustained. Misbalance leads to the biogeochemical succession on the large time scale. Modified from Zavarzin GA (2004) Lekcii po Prirodovedcheskoi Mikrobiologii (Lectures in Environmental Microbiology). Moscow: Nauka.

Figure 1 Interlink between the cycles of the main biogenic elements. Cycle of Corg makes the main driving force of machinery coupled to the cycles of biomass constituents Norg and Porg and catabolic cycles of oxygen, sulfur, and iron. Cycles of these elements are coupled to reservoirs of inorganic matter in the geosphere. For each time period approximate material balance should be sustained. Misbalance leads to the biogeochemical succession on the large time scale. Modified from Zavarzin GA (2004) Lekcii po Prirodovedcheskoi Mikrobiologii (Lectures in Environmental Microbiology). Moscow: Nauka.

liberated is equivalent to the total Corg extracted from the system into the biomass and the reduced products of its decomposition. The link between reservoirs of inorganic and organic carbon is performed by enzyme carboanhydrase (CA) in the reaction CO2 + H2O + CA $ H2O-CA-CO2 $ HCO3~ + H++ CA. In cyanobacteria, CA and RubisCO are integrated into the structural unit carboxysome. In eukaryotes, intracellular localization of enzymes is different. CA is responsible for CO2 evolution during respiration. The production is measured either by O2 production in water systems or by 1 C-bicarbonate assimilation. The cycle of Corg is linked to the reservoir of C^^ with a strong influence of the calcium cycle.

Photosynthesis is the dominating process in production. Primary production is proportional to the illuminated surface or more precisely to the density of chlorophyll, with an approximate ratio of annual assimilation 145kgCorg per kg of chlorophyll in terrestrial boreal ecosystems. Formation of Corg-pool occurs through several steps. The first one is gross primary production (GPP), counterbalanced by photorespiration in which approximately half of carbon is lost. Netto primary production (NPP) is calculated on the annual basis of the growing season for a plant. Evidently for algae with a short life cycle, the concept is different. Corg balance in the ecosystem is different and includes losses by the respiration of decomposers; it is referred to as Netto ecosystem production (NEP). Optimal conditions for photosynthesis and destruction are different:

destruction has higher optimal temperature than photosynthesis, and different dependence on water, being suppressed by the excess of water, causing anaerobiosis. This causes zonal variance for biomes. The accumulation of Corg on the decadal scale is designated as Netto biome production (NBP) for which accumulation of nondecom-posed Corg as humic substances and peat is the main parameter. For marine ecosystems, 'dissolved Corg' substitutes soil humus. The recalcitrant Corg of humic substances has a residence time of about millennia. It is converted into carbon of sedimentary rocks known as 'kerogen', which makes the main reservoir of reduced carbon on the planetary scale with a residence time of more than millions of years, depending on geological recycle. The reservoir of kerogen is sufficient to balance oxygen in marine sulfates and iron oxides deposits. Only 5% of the total oxygen produced remains in the transi-tionary reservoir of the atmosphere.

From a brief description of Corg-cycle, it is evident that the residence time in reservoirs is to be included into consideration. Seasonal variations in CO2 fluxes are illustrated by the annual oscillations of atmospheric CO2 in the continental Northern Hemisphere with an amplitude of about 20 ppm (parts per million) in Hawai and increasing in higher latitudes. In the oceanic Southern Hemisphere, oscillations are smoothened by the carbonate/bicarbonate system of the ocean.

Destructive pathways begin by decomposition of the dead biomass. Transition from living to dead biomass is accompanied by autolysis, which liberates part of the organic matter. For cyanobacteria and algae, lysis induced by viruses or phagi is quite important. Density of population is important, and below 105 cells per milliliter, phagolysis is ineffective. Lysis produces two components: dissolved organic compounds (DOCs) and particulated organic compounds (POCs), which consist mainly of structural components of the cell. Osmotrophic microorganisms can immediately use DOC; the threshold depends on the dilution with 1-10 mgl-1 still utilizable depending on the inflow. POC is to be hydrolyzed by hydrolytic exoenzymes before osmotrophic organisms can utilize it - bacteria in the sea or fungi in terrestrial ecosystems. Destructive pathways are formed by organotrophic organisms, which traditionally are named heterotrophs. This term is imprecise since it refers to the assimilative pathway leading at the end to the secondary production. There are three main metabolic pathways for Corg: proteolytic, saccharolytic, and lypolytic, according to the composition of biomass. The Winogradsky rule (1896) says that each natural compound has its specific microbial decomposer. The number of species of prokaryotes exceeds 5000 of cultivated and 2x10 clones of noncultivated. That gives sufficient functional diversity to perform biogeochemical essential reactions. As a result, specific trophic groups of organisms characterized by the utilizable substrate (e.g., cellulolytic, or lypolytic, or lignin-decomposing fungi), appear. The set of these organisms makes the functional biodiversity in the trophic system, which should make a complete community for each habitat. In the terrestrial environment, mycelial fungi are most important. Wood consists of 20-30% lignin, which is decomposed by fungi and that gives the lower limit of their involvement in terrestrial Corg-cycle as 1/3-1/4 of CO2 producers. Adding cellulose decomposition would at least double their contribution.

In the presence of O2, aerobic organisms regenerate approximately one-third of Corg in secondary production with CO2 as the product of respiration. Consumption of O2 in the dark is by the usual estimation of respiration by the so-called biochemical oxygen demand (BOD) test.

In an anoxic environment, a cascade of reactions begins with fermentation, which is also the main pathway for many hydrolytic decomposers. As products, a mixture of organic acids and H2 appears, and this is the reason why this stage is designated as acidogenic or hydrogen producing. Organic acids as nonfermentable compounds can be utilized only with an external oxidant, such as nitrate or ferric iron, sulfate, or CO2. A cascade of anaerobic reactions makes a community function as an entity with an integrated trophic network.

Without an external oxidant, anaerobic decomposition is completed by methanogenesis, a process which dominates in terrestrial mires and lake mud. In the sea, it takes place in deep layers of sediments, when sulfate is exhausted from interstitial water. Methanogens make a group of Euryarchaeota usually named as Methano-... . There are three pathways for methanogenesis: either hydrogeno-trophic with H2 + CO2, or acetoclastic, or methylotrophic for C-1 compounds. Acetoclastic pathway dominates in Corg-abundant environment, for instance, in methane tanks. Methylotrophic methanogens develop noncompeti-tive pathways in saline environment, while they do not compete with sulfate reducers. Hydrogenotrophic metha-nogens can use endogenous H2 formed by reaction of water with superheated rocks and belong to hyperthermophiles, for example, Methanobacterium fervidus, which develops at temperatures over 100 °C. More important is the role of hydrogenotrophic methanogens in community, where they act as H2-sink. They establish H2-concentration below 105ppm and this allows us to oxidize acetate and other nonfermentable substrates in cooperative action with H2-producing syntrophic organisms. Biogenic methane is identified by its isotopically light composition. Most of methane is 13C-depleted.

Methane either remains in the sediments or escapes into the oxic zone where it is oxidized with O2 by a specific group of methanotrophs. Under geologically favorable conditions, methane is stored in sedimentary rocks. Another possibility is the formation of crystallohydrates, which at appropriate hydrostatic pressure and low temperature make an ice-like cover for deep methane. At present, about 500 Mt yr~ of methane comes into the atmosphere, where it is oxidized photochemically. Many times more than this quantity is oxidized by methanotrophs, which form an oxidative filter on the path of CH4 to the atmosphere. The genera of methanotrophs are designated as Methylo- Oxidation of CH4 includes its enzymatic transformation in C-1 compounds in the cell by a special metabolic pathway and thus methanotrophs represent a specialized group of the Proteobacteria. In the ocean, methane is oxidized by anaerobic consortia ofmethanogens with sulfate-reducing or denitrifying bacteria. The microbial cycle of CH4 is most important for the biosphere.

Involvement of oxidized N, Fe, and S compounds as oxidants conjugates Corg-cycle with cycles of other elements. Transition from oxic to anoxic zone favors retainment of nondecomposed organic matter and leads to formation of oil and gas deposits in aquatic environments and coal in terrestrial ecosystems.

Particulated components including bodies of bacteria are consumed by phagotrophic Protists or/and by zoo-trophic multicellular animals. The trophic chains of animals are arranged into a trophic pyramid with a number of levels, including herbivorous and carnivorous. The size of the prey determines the nutritional pyramid. Animals that use filtration for nutrition are important in aquatic environment, keeping the density of microorganisms on the threshold level of about 10 cells ml-1. The total amount of bacteria in the active zone of the ocean and soil is at least of the order 1028, with the biomass of each cell about 10- g.

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